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    <title>Document</title>
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<body>
    <script>
        let num1
        let num2
        function sum(num1, num2) {
            num1 = num1 || 0
            num2 = num2 || 0
            return num1 + num2
        }
        num1 = +prompt(`请输入第一个数字`)
        num2 = +prompt(`请输入第二个数字`)
        let getsum = sum(num1, num2)
        document.write(getsum)
    </script>
</body>

</html>